The Roush engines all arrived pre mapped by Roush for MG, However, there are now none available. You may be lucky to find a Roush S/C for sale, Stal 108 had one for sale but I think that went to a none 260 owner via ebay. Linden has a very nice ZT 385 Roush Supercharged car for sale now for 12K. Get that and you will have one of only 6 available.
You may think 12K steep, but £7500 to £8500 for a supercharger then 6K to 8K for a car make 12K cheap. If you have the money buy Lindens.